Pursuant to our discovery (1971) of a photochemical route to fluoroimidazoles, we have found that various fluoro analogs of bioimidazoles (histamine, histidine, etc) show a variety of useful properties as substrates and inhibitors. For example, 2-fluoro-L-histidine not only is incorporated into microbial and animal protein in place of histidine, but also serves as an antibacterial, antiviral and antileukemic agent. Recent studies with labelled amino acids reveal that this analog also limits the uptake of several other amino acids into the cell. These, and other, biological properties appear to be specific for the 2-fluoro isomer. 5-Fluoro-Im-TRH failed to bind to rat pituitary cells in vitro and failed to stimulate prolactin release. On the other hand, microinjection of this compound into the rat hypothalamus results in strong elevations in heart rate and blood pressure. Thus, the long-sought separation of the pituitary and CNS activities of TRH have been achieved by means of a fluoro analog. Additional analogs are being synthesized in the hope of achieving stability to hydrolytic breakdown without loss of selectivity. Fluoroimidazoles are obtained by photochemical replacement of amino groups and aminoimidazoles are generally prepared by reduction of arylazoimidazoles. Thus, 4-fluoro-2-aminoimidazole-5-carboxamide was prepared and was converted into the 2,4-difluoro derivative. Surprisingly, the isomeric 2-fluoro-4-aminoimidazoles are far less stable and must be stored at very low temperatures. The N-trifluoroacetyl derivative, however, is quite stable in the solid state at 25 C. When the latter compound is dissolved at pH 7.4 and 37 C, the trifluoroacetyl group is lost spontaneously (t1/2 = 3 hr) and the potential antiviral agent is released in situ. In parallel with the broad-spectrum antiviral activities already shown by 4-fluoroimidazole-5-carboxamide, the new compounds will also be investigated for such activities.